Servo brake booster with two differentiated fixed and variable jumps

ABSTRACT

A pneumatic brake booster ( 10 ) of the type comprising a moving piston ( 22 ) urging an actuating rod ( 34 ) of a master cylinder ( 28 ) in response to actuation of a plunger ( 56 ) of which a front end, passing through the piston ( 22 ) and forming a first feeler ( 62 ) can, when the plunger ( 56 ) is in the extreme position, penetrate a reaction disc ( 44 ) inserted between the piston ( 22 ) and the actuating rod ( 34 ) to transmit to the plunger ( 56 ) the reaction force of the master cylinder ( 28 ), characterized in that it comprises a second feeler ( 68 ) which can, when the control rod ( 46 ) is actuated at a rate higher than a determined rate, be pushed by the plunger ( 56 ) and immobilized by a one-way engagement device ( 70 ) in an axial position that varies with respect to the piston ( 22 ), then be driven by the piston to push the second feeler ( 68 ) into the disc ( 44 ) so that the first feeler ( 52 ) applies a braking force more rapidly.

[0001] The invention relates to a pneumatic brake booster for actuatinga brake master cylinder of a motor vehicle.

[0002] The invention relates more particularly to a pneumatic brakebooster for actuating a brake master cylinder of a motor vehicle, of thetype comprising a rigid casing in which there can move a transversepartition sealingly delimiting a front chamber subjected to a first,engine depression, pressure and a rear chamber subjected to a secondpressure that varies between the engine depression and atmosphericpressure, of the type which comprises a moving piston secured to themoving partition, a front face of which can act on a rod for actuationof the master cylinder, of the type comprising a rod for control of thebooster, moving in the piston selectively as a function of an axialinput force exerted forwards against a return force exerted on the rodby a return spring (50), of the type comprising a plunger which isarranged in front of the control rod in the piston and which at its rearend comprises at least one annular rear seat for a three-way valve whichcan move progressively between a position in which, with the control rodat rest, the front chamber and the rear chamber are in communication,and a position in which, with the control rod actuated, the secondpressure in the rear chamber increases, the valve placing the rearchamber (18) in communication with atmospheric pressure, and of the typecomprising a first feeler forming the front end of the plunger andpassing through the piston which, when the control rod is in the restposition, is arranged a first determined jump distance away from areaction disc inserted between the rod for actuation of the mastercylinder and the front face of the moving piston, and which is able,when the control rod is actuated at a rate lower than a determined rate,to be made by the plunger to cover the first jump distance then topenetrate the reaction disc in such a way as to transmit to the plungerand to the control rod the reaction force of the master cylinder.

[0003] Numerous examples of conventional boosters of this type areknown.

[0004] In such a booster, the distance separating the first feeler fromthe reaction disc is known as the “jump distance” and corresponds to thetheoretical distance that the first feeler has to cover before thedriver of the vehicle feels the reaction force of the master cylinder.

[0005] In practice, when there is clearance between the first feeler andthe reaction disc, this distance corresponds to the distance separatingthe first feeler from the reaction disc, but it may also, when thereaction disc is initially decompressed in contact with the reactiondisc, correspond to the travel over which the first feeler is likely tocompress the reaction disc until the latter is fully compressed.

[0006] In an extreme braking situation for which maximum braking forceis exerted on the control rod, actuation of the control rod causesmovement of the plunger comprising the first feeler, and this causes thethree-way valve to open wide and the rear chamber to be placed atatmospheric pressure. This results in a forwards movement of the movingpartition, and the end of the plunger forming the first feelerpenetrates the reaction disc made of elastomer which is secured to therear face of the moving piston, compressing it.

[0007] Thus, the force exerted on the rod for actuation of the mastercylinder when the control rod is at the end of its travel is the resultof the boost force which is brought about by the pressure differenceacross the moving wall and the force exerted by the plunger forming afeeler on the reaction disc. The driver of the vehicle also feels thebraking reaction force, which is transmitted from the master cylinder tothe plunger via the reaction disc.

[0008] Now, it has been found that a good many drivers, when confrontedwith an emergency braking situation, underestimate the risks involvedand, having braked sharply, release their braking force even thoughmaintaining a braking force is essential in order to avoid an accident.

[0009] What happens is that, in the case of an extreme braking situationaccompanied by a swift movement of the control rod, the plunger may comeinto contact with the reaction disc and transmit to the driver a feelingof maximum braking before the pressure difference across the front andrear chambers reaches its maximum, and this may lead the driver torelease his force, even though he ought to maintain it in order tobenefit from maximum braking force.

[0010] In order to remedy this drawback, a booster has been proposedwhich comprises a first feeler which is slidably mounted with respect tothe plunger to penetrate the reaction disc and which can then, when thecontrol rod of the booster is actuated at a determined rate, be lockedwith respect to the moving piston so as to maintain maximum brakingforce on the rod for actuation of the master cylinder via the reactiondisc even though the driver may have partially released his force.

[0011] This design has the disadvantage of entailing a significant forceon the part of the driver, because in order for the feeler to penetratethe reaction disc at the end of its travel, the actuating force has tobe exerted against the reaction force of the master cylinder.

[0012] This is particularly penalizing in the case of emergency braking,that is to say when a braking force is applied swiftly when the durationof actuation needs to be as short as possible.

[0013] The invention overcomes the drawbacks of the two aforementioneddesigns by proposing a booster according to the first design to which isadded a second feeler which, during swift application, can be pushed bythe plunger and locked axially with respect to the piston, then bepushed by the piston into the reaction disc so that swift application ofa braking force can be achieved, with a new jump distance of variablevalue greater than the initial value, and encountering only a reducedantagonistic force transmitted by the reaction disc so that applicationof the braking force can be achieved more swiftly.

[0014] To this end, the invention proposes a booster as describedpreviously, characterized in that it comprises a tubular bushing whichis slidably mounted on the plunger, which comprises a front portionwhich passes through the piston and an end of which comprises a secondfeeler, and which is capable, when the control rod is actuated at a ratehigher than a determined rate, of being pushed by the plunger andimmobilized with respect to the piston by a one-way engagement device inan axial position that can vary depending on the rate at which thecontrol rod is actuated, then of being driven by the piston to cause thesecond feeler to penetrate the reaction disc, so as to allow the firstfeeler to apply a rapid braking force against an antagonistic reactionforce of the master cylinder which is reduced along a second, variable,jump distance greater than the first jump distance.

[0015] According to other features of the invention:

[0016] the bushing comprises a rear tubular portion which is slidablymounted in a complementary rear bore of the piston,

[0017] the one-way engagement device comprises a key which is mounted ina recess of the piston passing through the piston at right angles to itsaxis and of which a roughly annular part surrounds the bushing withclearance and is capable, when the input force exerted on the controlrod is at a rate higher than the determined rate, of being driven by themoving piston to rock about an axis which, on the whole, is transversal,so as to collaborate with the periphery of the rear tubular portion ofthe bushing to immobilize the said bushing in the variable axialposition,

[0018] the rear tubular portion of the bushing has a constant diameterso that the roughly annular part of the key immobilizes the bushing bywedging,

[0019] the key comprises a roughly L-shaped lower part which extendsradially from its annular part and which is returned elastically againstan abutment face of the casing of the piston by a return spring insertedbetween a front face of the recess of the piston and a front face of anupper part of the key,

[0020] the second feeler comprises an annular front face which iscoaxial with the bushing and which has an outside diameter greater thanthat of the rear portion of the bushing,

[0021] the reaction disc is housed in a cup coaxial with the piston,which is secured to the rear end of the actuating rod and a rear tubularpart of which is slidably mounted in a coaxial annular groove of thefront face of the piston,

[0022] the second feeler is formed as one with the front end of thebushing,

[0023] the second feeler is produced in the form of an annulus which isslidably mounted on the cylindrical front end of the bushing and whichcan be pushed into the reaction disc by a shoulder front face of thebushing.

[0024] Other features and advantages of the invention will becomeapparent from reading the detailed description which follows, forunderstanding of which reference will be made to the appended drawingsin which:

[0025]FIG. 1 is an overall view in axial section of a pneumatic brakebooster produced according to the invention,

[0026]FIG. 2 is a detail view in axial section of the booster of FIG. 1,depicted with the control rod in a rest position,

[0027]FIG. 3 is a detail view in axial section of the booster of FIG. 1,depicted in a position of slow application of a braking force to thecontrol rod,

[0028]FIG. 4 is a detail view in axial section of the booster of FIG. 1depicted during high-speed application of a braking force to the-controlrod in a first stage of start of actuation of the control rod,

[0029]FIG. 5 is a detail view in axial section of the booster of FIG. 1depicted during high-speed application of a braking force to the controlrod in a second, intermediate, stage of actuation of the control rod,

[0030]FIG. 6 is a detail view in axial section of a booster in analternative form, depicted with the control rod in a rest position.

[0031] In the description which will follow, identical referencenumerals denote parts which are identical or have similar functions.

[0032] By convention, the terms “front”, “rear”, “upper” and “lower”refer respectively to elements or positions directed respectively to theleft, to the right, to the top or to the bottom of the figures.

[0033]FIG. 1 depicts the entirety of a pneumatic brake booster 10 for amotor vehicle.

[0034] In the known way, the pneumatic booster 10 comprises a rigidcasing 12 inside which is movably mounted a transverse partition 14which sealingly delimits a front chamber 16, subjected to a firstpressure “P₁” the value of which is equal to the vehicle enginedepression, and a rear chamber 18 subjected to a second pressure “P₂”.The second pressure “P₂” is able, as will be described later, to varybetween the value of the engine depression “P₁” and the value ofatmospheric pressure “P_(a)”.

[0035] The front chamber 16 is supplied with pressure “P₁” by adepression pipe 20 which is connected, for example, to an inlet manifold(not depicted) of an engine of the vehicle.

[0036] The pneumatic booster 10 comprises a moving piston 22 which issecured to the moving partition 14. Inside the casing 12, the movingpartition 14 is returned elastically backwards by a return spring 24which bears against the casing 12 and against a front face 26 of themoving piston 22.

[0037] The booster 10 is coupled to a brake master cylinder 28 which isalso connected to a braking circuit of the vehicle via hydraulic pipes30 and 32.

[0038] In particular, the front face 26 of the moving piston 22 iscoupled to an actuating rod 34 of the master cylinder 28 via a cup 36coaxial with the piston 22 which is secured to the rear end 38 of theactuating rod 34 and a rear tubular part 40 of which is slidably mountedin a coaxial annular groove 42 made in the front face 26 of the piston22. The cup 36 houses a reaction disc 44 made of elastomeric material,the function of which will be described later on.

[0039] The piston 22 comprises a control rod 46 which is, for example,connected to a brake pedal (not depicted) of the vehicle via a couplingsleeve 48 which is arranged at its rear free end. The rod 46 is able tomove in the moving piston 22 selectively as a function of an axial inputforce exerted forwards on the control rod 46. The actuating force isexerted against a return force exerted on the rod 46 by a return spring50 interposed between the moving piston 22 and the control rod 46.Sealing against dust and foreign bodies of the rear end of the movingpiston 22 is afforded by a boot 51 which is mounted on the rear end 53of the casing 12 and through which the control rod 46 passes. The other,front, free end of the control rod 46 is shaped into a ball 52 which ishoused in a complementary housing 54 of a roughly cylindrical plunger 56which is slidably mounted in the moving piston 22.

[0040] At its rear end, the plunger 56 comprises at least one rearannular seat 58 of a three-way valve 60 which can move progressivelybetween a position in which, with the control rod 46 at rest, the frontchamber 16 and the rear chamber 18 are in communication, and a positionin which, with the control rod 46 actuated, the second pressure “P₂” inthe rear chamber 18 increases, the valve 60 placing the rear chamber 18in communication with atmospheric pressure “P_(a)”.

[0041] As the way in which the three-way valve 60 works is known fromthe state of the art, this will not be described in any greater detailin this description.

[0042] In the known way, the booster 10 comprises a first feeler 62,forming the front end of the plunger 56 and passing through the piston22. More specifically, the first feeler 62 opens into the front face 26of the piston 22 facing the reaction disc 44 which is arranged in thecup 36.

[0043] As illustrated more particularly in FIG. 2, the plunger 56therefore comprises a front portion 61, the end of which forms the firstfeeler 62, an intermediate portion 63 of a diameter greater than thefront portion, and a rear portion 65 the rear end of which forms theseat 58 of the valve.

[0044] When the control rod 46 is in the rest position depicted in FIG.2, the first feeler 62 is arranged a first determined jump distance “d1”away from the reaction disc 44. When the control rod rod [sic] 46 isactuated at a rate lower than a determined rate, the first feeler 62 iscapable, as will be seen later on, of accompanying the piston 22 overthe first jump distance “d1” until, with the piston 22 having fullycompressed the reaction disc 44 in the cup 36, this disc comes intocontact with the first feeler 62 and transmits to the control rod 46 thereaction force of the master cylinder 28 as depicted in FIG. 3.

[0045] According to the invention, the booster 10 comprises a tubularbushing 64 which is mounted to slide on the plunger 56 which comprises afront portion 66 passing through the piston 22 and one end of whichcomprises a second feeler 68 which is capable, when the control rod 46is actuated at a rate higher than the determined rate, of being pushedby the plunger 56 and immobilized with respect to the piston 22 by aone-way engagement device 70 in an axial position that can varydepending on the rate of actuation of the control rod 46, then of beingdriven by the piston to make the second feeler 68 penetrate the reactiondisc 44, as depicted in FIG. 4, so as to allow the first feeler 62 toapply a rapid braking force against an antagonistic reaction force ofthe master cylinder 28 which is reduced along a second, variable, jumpdistance “d2” greater than the first jump distance “d1”, as depicted inFIG. 5.

[0046] According to a first embodiment which is depicted in FIGS. 1 to5, the second feeler 68 is formed as one with the front end 67 of thebushing 64.

[0047] According to a second alternative form of embodiment which isdepicted in FIG. 6, the second feeler is produced in the form of anannulus 68 which is slidably mounted on the cylindrical front end 67 ofthe bushing 64 and which can be pushed into the reaction disc 44 by ashoulder front face 69 of the bushing 64.

[0048] In both embodiments of the invention, the bushing 64 is slidablymounted in the piston 22 and is preferably fully coaxial with theplunger 56. For that, the bushing 64 comprises a rear tubular portion 72which is slidably mounted in a complementary rear bore 74 of the piston22.

[0049] This arrangement is not a limitation on the invention, and thesecond feeler 68 could be not coaxial with the plunger 56 provided thatthe front portion 66 of the bushing 64 which carries the second feeler68 is coaxial with the plunger 56.

[0050] Advantageously, the second feeler 68 has an outside diametergreater than that of the rear portion 72 of the bushing and somewherebetween that of the reaction disc 44 and that of the first feeler 62.

[0051] Furthermore, the rear portion 72 of the bushing 64 comprises abore 76 which is of a diameter that corresponds to that of theintermediate portion 63 of the plunger 56.

[0052] The one-way engagement device 70 comprises a key 78 which ismounted in a recess 80 of the piston 22 passing through the piston 22 atright angles to its axis A, and of which a roughly annular intermediatepart 82 surrounds the bushing 64, with clearance. The key 78 comprises aroughly L-shaped lower part 84 which extends radially from the annularpart 82, and a roughly radial upper part 85. The lower part 84 iselastically returned against an abutment face 86 of the casing 12 of thebooster 10, depicted in FIG. 1, by a return spring 88 of the key 78which spring is inserted between a front face 90 of the recess 80 of thepiston 22 and a front face 92 of the upper part 85.

[0053] More particularly, a horizontal branch 96 of the L-shaped lowerpart 84 is more particularly intended to come into abutment against theabutment face 86 of the casing 12 of the booster 10.

[0054] Advantageously, as can be seen in FIG. 1, the abutment face 86 ofthe casing 12 is, for example, carried by a ring 98 which is housed inthe cylindrical casing of the booster 14.

[0055] As illustrated in FIG. 4, the key 78 is capable, when the inputforce is inserted on the control rod 46 at a rate higher than thedetermined rate, of being driven by the moving piston 22 to rock aboutan axis which is on the whole transversal so as to collaborate with theperiphery of the rear tubular portion 72 of the bushing 64 to immobilizethe said bushing 64 with respect to the piston 22 in a variable axialposition.

[0056] For this, the rear tubular portion 72 of the bushing 64 has aconstant diameter so that the roughly annular intermediate part 82 ofthe key 78 immobilizes the bushing 64 by wedging.

[0057] More particularly, with the part 82 hollowed out at a diametergreater than that of the rear portion 72 of the bushing 64, contactwhich occurs between the annular part 82 of the key 78 and the peripheryof the rear tubular portion 72 of the bushing 64 is contact of theroughly point-contact type.

[0058] In the position of rest depicted in FIG. 2, a shoulder front face102, which delimits the intermediate 63 and rear 65 portions of theplunger 56, forms an axial clearance “J” with a shoulder face 104 of therear portion 72 of the bushing 64.

[0059] When a braking force is applied swiftly to the plunger 56, theshoulder front face 102 moves forwards, absorbing the clearance “J” andencounters the shoulder face 104, and this has the effect of pushing thebushing 64 back as depicted in FIG. 4.

[0060] This configuration advantageously allows a great many axialpositions of immobilization of the bushing to be proposed, each positionbeing dependent upon the axial position to which the bushing 64 ispushed back with respect to the key 78 when the latter rocks.

[0061] Thus, during swift application of a braking force to the plunger56 following actuation of the control rod 46, the piston 22 first of alladvances less quickly than the plunger 46 because of the delay inbalancing the pressures in the front 14 and rear 16 chambers of thebooster. As a result, the swifter the application of the braking force,the more the bushing 64 is pushed back by the plunger 46 before, withthe piston 22 advancing in its turn and the key 78 leaving its restingplace on the abutment face 98 of the casing, the key 78 rocks.

[0062] In this configuration, the booster 10 can operate in differentconfigurations which have been depicted in FIGS. 2 to 5.

[0063] When the control rod 46 is in the rest position depicted in FIG.2, the horizontal branch 96 of the lower part 84 of the key 78 restsagainst the abutment face 86 of the casing 12 of the booster 10 and theupper part 85 of the key 78 rests against the rear face 91 of the recess80.

[0064] A transverse pin 106 which passes through an aperture (notdepicted) in the bushing 64 and the plunger 56 is, when the rod 46 is inthe rest position, resting against a front face of the annularintermediate part 82 of the key 78 to determine the rest position of theplunger 56, the first feeler 62 of which forms, with the reaction disc44, the first jump distance “d1”. The shoulder front face 102 of theplunger 56 forms the clearance “J” with the shoulder face 104 of therear portion 72 of the bushing 64.

[0065] Thus, as illustrated by FIGS. 3 to 5, when an input force isapplied forwards along extreme travel of the control rod 46, the lowerpart 84 of the key 78 leaves its point of rest against the abutment face86. As a result, the key 78 rocks about an axis which on the whole istransversal, and in the anticlockwise direction, to collaborate with theperiphery of the bushing 64.

[0066] If the input force is applied relatively slowly, that is to sayin the case of normal braking depicted in FIG. 3, the pressures “P₁” and“P₂” in the front and rear chambers of the booster 10 balance each otherat roughly the same speed as the speed at which the plunger 56 advancesunder the effect of actuation of the control rod 46. As a result, theplunger 46 and the piston 22 advance at roughly the same rate. As soonas the piston 22 advances, the lower part 84 of the key 78 leaves itsresting place against the abutment face 86 and this causes the key 78 torock, the intermediate part 82 of which key drops on to the periphery ofthe front portion 66 of the bushing 64, immobilizing it. The secondfeeler 68, immobilized, accompanies the piston 22 without penetratingthe reaction disc 44. The piston 22 compresses the reaction disc 44 andthe latter deforms to fill the cavity formed in the bushing 64 by thejump distance “d1”. The distance “d1” is covered when the reaction disc44 can no longer compress in contact with the first feeler 62.

[0067] By contrast, if the input force is applied at a rate higher thana determined rate as depicted in FIG. 4, that is to say in the case ofemergency braking, the pressures “P₁” and “P₂” in the front and rearchambers of the booster 10 balance at a rate which is lower than therate at which the plunger 56 advances under the effect of actuation ofthe control rod 46.

[0068] As a result, according to a first stage of start of actuation ofthe control rod 46, which stage is depicted in FIG. 4, the plunger 56,which slides forward with respect to the piston 22, first of all pushesback the bushing 64 via its shoulder front face 102 to an axial positionthat can vary and the two feelers 62 and 64 advance simultaneously. Thepiston 22 starts to move in its turn and the key 78 drops down on to theperiphery of the rear portion 72 of the bushing 64, locking it in thisvariable position which depends on the rate at which the plunger 56 wasactuated before the key 78 dropped on to the periphery of the bushing64.

[0069] It should be pointed out that, from this moment on, the variableaxial position of the second feeler 68 with respect to the piston 22 isdetermined in a stable manner by the wedging of the key 78 on thebushing 64.

[0070] Next, in a second, intermediate, stage of actuation of thecontrol rod, which stage is depicted in FIG. 5, the piston 22 movesunder the effect of the increase in pressure in the rear chamber 18 ofthe booster. As a result, the second feeler 68, moved by the bushing 64locked with respect to the piston 22, penetrates deeper into thereaction disc 44 while the first feeler 62 remains more or lessimmobile. As a result, the second feeler 68 locally compresses thereaction disc and this has the effect of locally decompressing thereaction disc 44 at the level of the first feeler 62.

[0071] As the jump distance is characterized by the distance whichseparates the rest position of the first feeler 62 and the position ofthe feeler 62 in which it compresses the reaction disc 44, the feeler 62can therefore still cover a variable second jump distance “d2”determined initially by the axial position at which the second feeler 68is immobilized with respect to the piston 22, which distance is greaterthan the previously mentioned jump distance “d1”.

[0072] It will be understood that the position of the second feeler 68and, in particular, its penetration into the reaction disc 44, dependson the distance by which the bushing 64 is pushed back before the key 78rocks, and therefore on the rate at which the braking force is appliedto the control rod 46.

[0073] According to a final third stage of actuation of the control rod46, the first feeler 62 can therefore locally recompress the reactiondisc 44 along the distance “d2” until the said disc 44 can no longer becompressed. At the end of this third stage, the booster is therefore ina configuration similar to the one depicted in FIG. 4, except that, thepiston 22 has advanced in the casing 12 of the booster 10.

[0074] In this instance, this second distance “d2” is appreciablygreater than the distance separating the first feeler 62 from the secondfeeler 68, but it would be possible for the first feeler 62 to compressthe reaction disc 44 when it came into line with the second feeler 68,this all depending on the mechanical properties of the elastomer ofwhich the reaction disc 44 is made.

[0075] Whatever the case may be, the distance “d2” that can varyaccording to the travel with which the bushing 64 was pushed backinitially will be covered by the first feeler 62 in the presence of anantagonistic reaction force from the master cylinder which is reducedand variable because the reaction disc 44 will be kept more or lesscompressed at the second feeler 68 which is locked by the key 78.

[0076] Advantageously, the swifter the application of the braking force,the more the second feeler 68 will have been pushed back with respect tothe piston 22, and the deeper it will penetrate the reaction disc 44when moved by the moving piston 22.

[0077] This will have the effect of locally compressing the reactiondisc 44 still more and as a result, the reduced force to be supplied bythe driver in order for the first feeler 62 to overcome the antagonisticreaction force of the master cylinder 28 will be all the smaller.

[0078] It will be understood that, in the preferred embodiment of theinvention, a distance “12” separating the shoulder 104 of the bushing 64from the second feeler 68 needs to be kept smaller than a distance “11”separating the shoulder 102 of the plunger 56 from the first feeler 62,this being so that, at the end of the travel of the actuating rod 46,that is to say once the jump distance “d2” has been covered, the feeler62 will actually penetrate the reaction disc 44 to transmit the reactionforce from the master cylinder 28 to the driver of the vehicle.

[0079] It will also be understood that complete release of the controlrod 46, which causes the piston 22 to move back, causes the lower part84 of the key to return to rest against the abutment face 86 of the ring98 and therefore causes the bushing 64 to be unlocked in a configurationsimilar to that of FIG. 2.

[0080] The booster 10 is therefore characterized by two differentiatedjump distances, namely a first jump distance “d1” which is fixed andreserved for application of a braking force at a slow rate, and a secondjump distance “d2” which is variable, reserved for the application of abraking force at a fast rate.

[0081] The invention therefore advantageously allows modulation of theintensity of the braking force to be supplied in the context of swiftapplication and allows a braking force to be maintained after this swiftapplication.

1. Pneumatic brake booster (10) for actuating a brake master cylinder(28) of a motor vehicle, of the type comprising a rigid casing (12) inwhich there can move a transverse partition (14) sealingly delimiting afront chamber (16) subjected to a first, engine depression, pressure(P₁) and a rear chamber (18) subjected to a second pressure (P₂) thatvaries between the engine depression and atmospheric pressure (P_(a)),of the type which comprises a moving piston (22) secured to the movingpartition (14), a front face (26) of which can act on a rod (34) foractuation of the master cylinder, of the type comprising a rod (46) forcontrol of the booster (10), moving in the piston (22) selectively as afunction of an axial input force exerted forwards against a return forceexerted on the rod (38) by a return spring (50), of the type comprisinga plunger (56) which is arranged in front of the control rod (46) in thepiston (22) and which at its rear end comprises at least one annularrear seat (58) for a three-way valve (60) which can move progressivelybetween a position in which, with the control rod (56) at rest, thefront chamber (16) and the rear chamber (18) are in communication, and aposition in which, with the control rod (56) actuated, the secondpressure (P₂) in the rear chamber (18) increases, the valve (60) placingthe rear chamber (18) in communication with atmospheric pressure(P_(a)), and of the type comprising a first feeler (62) forming thefront end of the plunger (56) and passing through the piston (22) which,when the control rod (46) is in the rest position, is arranged a firstdetermined jump distance (d1) away from a reaction disc (44) insertedbetween the rod (34) for actuation of the master cylinder (28) and thefront face (26) of the moving piston (22), and which is able, when thecontrol rod (46) is actuated at a rate lower than a determined rate, tobe made by the plunger (56) to cover the first jump distance (d1) thento penetrate the reaction disc (44) in such a way as to transmit to theplunger (56) and to the control rod (46) the reaction force of themaster cylinder (28), characterized in that it comprises a tubularbushing (64) which is slidably mounted on the plunger (56), whichcomprises a front portion (66) which passes through the piston (22) andan end of which comprises a second feeler (68), and which is capable,when the control rod (46) is actuated at a rate higher than a determinedrate, of being pushed by the plunger (56) and immobilized with respectto the piston (22) by a one-way engagement device (70) in an axialposition that can vary depending on the rate at which the control rod isactuated, then of being driven by the piston to cause the second feeler(68) to penetrate the reaction disc (44), so as to allow the firstfeeler (62) to apply a rapid braking force against an antagonisticreaction force of the master cylinder (28) which is reduced along asecond, variable, jump distance (d2) greater than the first jumpdistance (d1).
 2. Booster (10) according to the preceding claim,characterized in that the bushing (64) comprises a rear tubular portion(72) which is slidably mounted in a complementary rear bore (74) of thepiston (22).
 3. Booster (10) according to the preceding claim,characterized in that the one-way engagement device (70) comprises a key(78) which is mounted in a recess (80) of the piston (22) passingthrough the piston (22) at right angles to its axis (A) and of which aroughly annular part (82) surrounds the bushing (64) with clearance andis capable, when the input force exerted on the control rod (46) is at arate higher than the determined rate, of being driven by the movingpiston (22) to rock about an axis which, on the whole, is transversal,so as to collaborate with the periphery of the rear tubular portion (72)of the bushing (64) to immobilize the said bushing (64) in the variableaxial position.
 4. Booster (10) according to the preceding claim,characterized in that the rear tubular portion (72) of the bushing (64)has a constant diameter so that the roughly annular part (82) of the key(78) immobilizes the bushing (64) by wedging.
 5. Booster (10) accordingto claim: 3 characterized in that the key (78) comprises a roughlyL-shaped lower part (84) which extends radially from its annular part(82) and which is returned elastically against an abutment face (86) ofthe casing (12) of the booster (10) by a return spring (88) insertedbetween a front face (90) of the recess (80) of the piston (22) and afront face (92) of an upper part (85) of the key (78).
 6. Booster (10)according to claim 2 characterized in that the second feeler (68)comprises an annular front face which is coaxial with the bushing (64)and which has an outside diameter greater than that of the rear portion(72) of the bushing (64).
 7. Booster (10) according to the precedingclaim, characterized in that the reaction disc (44) is housed in a cup(36) coaxial with the piston (22), which is secured to the rear end (38)of the actuating rod (34) and a rear tubular part (40) of which isslidably mounted in a coaxial annular groove (42) of the front face (26)of the piston (22).
 8. Booster (10) according to the preceding claim,characterized in that the second feeler (38) is formed as one with thefront end (67) of the bushing (64).
 9. Booster (10) according to claim1, characterized in that the second feeler (38) is produced in the formof an annulus which is slidably mounted on the cylindrical front end(67) of the bushing (64) and which can be pushed into the reaction discby a shoulder front face (69) of the bushing (64).